Portable power tool with an anti-vibration balancing device

ABSTRACT

A portable power tool having a rotating output shaft ( 13 ) journalled in two axially spaced bearings ( 15,16 ) and having at its forward end ( 13   a ) a working implement mounting device ( 19 ) and an anti-vibration balancing unit ( 20 ), wherein an auxiliary balancer ( 26 ) is provided at the rear end ( 13   b ) of the output shaft ( 13 ) for dynamic balancing of the rotating parts including working implement ( 17 ) mounted on the output shaft ( 13 ) at operation of the power tool.

[0001] The invention relates to a portable power tool of the type having a rotating output shaft provided with an anti-vibration balancing device in the form: of a ball type balancing unit for dampening occurring unbalance in a working implement connected to the output shaft.

[0002] In prior art there are known power tools, see for instance U.S. Pat. No. 5,839,950, which are provided with a ball type balancing unit carried at the forward end of the output shaft so as to accomplish an automatic balancing of the vibration forces generated by a grinding wheel attached to the output shaft. By such an arrangement there is obtained a static balancing only, and since there is an inevitable axial distance between the grinding wheel and the balancing unit there will still be dynamic forces causing vibrations with respect to an axis of inertia of the tool. These vibrations cause an uncomfortable and sometimes a detrimental working situation for the tool operator.

[0003] It is a primary object of the invention to accomplish a portable power tool of the above type in which the static as well as the dynamic vibration forces are counteracted such that the working situation for the operator is improved.

[0004] Further objects and advantages of the invention will appear from the following specification and claims.

[0005] A preferred embodiment of the invention is hereinafter described in detail with reference to the accompanying drawing.

[0006] On the drawing:

[0007]FIG. 1 shows a side view of a pneumatic angle grinder according to the invention.

[0008]FIG. 2 shows a section through the forward end of the angle grinder shown in FIG. 1.

[0009]FIG. 3 shows a diagrammatic illustration of the balancing forces of the anti-vibration system according to the invention.

[0010] The power tool shown in the drawing figures is a pneumatic angle grinder comprising a housing 10, a rotation motor (not shown), a pressure air inlet valve 11 controlled by a lever 12, and an output shaft 13. The output shaft 13 has a forward end 13 a journalled in forward bearing 15 and a rear end 13 b journalled in a rear bearing 16. The output shaft 13 also fitted with a mounting device for a grinding wheel 17, and on the housing 10 there is mounted a safety guard 18 partly surrounding the grinding wheel 17 for protection against incidental grinding wheel desintegration.

[0011] The mounting device which is secured in a concentric threaded bore 19 in the output shaft 13 does not form part of the invention is therefore not described in further detail. This type of mounting device, however, is described in the above mentioned U.S. Pat. No. 5,839,950.

[0012] At its forward end 13 a, the output shaft 13 is provided with a balancing unit 20 of the type having a number of balls 21 freely movable along a circular race 22 for self adjustment and compensation for imbalance forces in the attached grinding wheel 17. The balancing unit 20 which is formed integrally with the output shaft 13 includes a cover washer 23 for sealing off the ball race 22 and maintaining the balls 21 in place.

[0013] The motor is connected to the output shaft 13 via an angle drive comprising a pinion 24 connected to the motor and a bevel gear 25 mounted on the output shaft 13 in a location between the forward bearing 15 and the rear bearing 16.

[0014] At its rear end 13 b, the output shaft 13 carries an auxiliary ball type balancer 26 which comprises a ring element 27 defining a circular path for a number of balls 28, and a mounting screw 29 engaging a central threaded bore in the output shaft 13 for securing the ring element 27 to the output shaft 13. The auxiliary balancer 26 is located behind the rear shaft bearing 16 and is intended to dampen the dynamic vibration forces not being taken care of by the forward balancing unit 20. The forward balancing unit 20 is effective in accomplishing a static balancing of the grinding wheel 17, i.e. generating a counteracting oppositely directed force equal to the imbalance force of the grinding wheel 17, but since this counteracting force is not applied in the same plane as the imbalance force of the grinding wheel 17 there will be a moment acting on the output shaft 13 causing vibrations.

[0015] As illustrated in FIG. 3, the grinder has a principal axis of inertia J_(xx), and the vibratory moment resulting from the imbalance force F₁ developed in the grinding wheel 17 and the counteracting compensating force F₂ provided by the balancing unit 20 at a distance S₁ makes the entire grinder oscillate with respect to the principal axis of inertia J_(xx).

[0016] To compensate for the vibratory moment, the auxiliary balancer 26 is located at the rear end of the output shaft 13, behind the rear bearing 16, to generate a dampening force F₁ acting at a distance S₂ from the compensating force F₂. Due to the fact that the distance S₂ is much larger than the distance S₁, the balancing force F₃ needed from the auxiliary balancer 26 is considerably smaller than the force F₁, because F₁ * S₁ shall equal F₃ * S₁. See FIG. 3.

[0017] By locating the auxiliary balancer 26 at as long as possible a distance from the balancing unit 20 the required balancing force F₃ and, hence, the size of the auxiliary balancer 26 can be kept small. 

1. Portable power tool with an anti-vibration balancing device, comprising a housing (10), a rotation motor, an output shaft (13) journalled in the housing (10) and having a rear end (13 b), and a forward end (13 a) provided with a working implement mounting device (19) as well as a ball type balancing unit (20), and a power transmission (24,25) coupling the motor to said output shaft (13), characterized in that an auxiliary ball type balancer (26) is secured to said rear end (13 b) of said output shaft (13) for accomplishing a dynamic balancing of said output shaft (13) at operation with an unbalanced working implement (17) attached to said working implement mounting device (19).
 2. Power according to claim 1 , wherein said output shaft (13) is journalled in a forward bearing (15) located close to said forward end (13 a), and a rear bearing (16) axially spaced from said forward bearing (15), said balancing unit (20) is located in front of said forward bearing (15), whereas said auxiliary balancer (26) is located behind said rear bearing (16).
 3. Power grinder according to claim 1 or 2 , wherein said power transmission (24,25) comprises an angle drive including a pinion (24) connected to the motor (11), and a bevel gear (25) secured to said output shaft (13) at a location between said forward bearing (15) and said rear bearing (16).
 4. Power grinder according to claim 3 , wherein said forward end (13 a) of said output shaft (13) and said balancing unit (20) extends out of said housing (10), and said rear end (13 b) of said output shaft (13) and said auxiliary balancer (26) are located inside said housing (10). 